Could someone please explain gimbal lock?
I gather that it's an inherent weakness of just storing three axial rotations (Quaternion, on the other hand, also evaluates orientation value).

(Per ideasman42's comment): We can easily see this for any object using Euler rotational system, by manipulating the rotation widget with transformation orientation set to Gimbal. It turns out to be easy to reproduce, by manipulating 2nd axis in the evaluation order (Z for XZY, or X for ZXY, etc.). I get the following condition on XYZ order by just manipulating Y axis:



Here's an article and a video for alternative explanations of the condition.